Glass ionomer cements have been used for more than 30 years for dental restorative treatments. Typically glass ionomer cements are reacted by mixing a powder part with a liquid part. The powder component typically comprises as essential or important component an acid-reactive filler (e.g. a fluoro alumino silicate glass). The liquid component typically comprises as essential or important components water, polycarboxylic acid and a complexing agent (e.g. tartaric acid) for adjusting the setting properties. Main advantages of glass ionomer cements are self-adhesion to tooth structure, fluoride release and the ability to be placed in one part (bulk-fill). The main disadvantage is the brittle nature and relatively low physical-mechanical properties compared to resin-based composite filling materials. Hence there have been various approaches to improve the latter, especially the flexural strength.
It was found that by increasing the overall content of polycarboxylic acid in comparison to the acid-reactive filler, the flexural strength could be improved. However, by increasing the amount of polycarboxylic acid contained in the liquid part, the liquid part became too viscous making it nearly impossible to adequately mix the powder and liquid component. To overcome this issue, a part of the polycarboxylic acid was put in dry form into the powder component. By doing this, however, it was realized that the storage stability of the product is negatively affected. Over time, humidity being present in the air starts to migrate into the powder component causing a glass ionomer reaction to start at least partially.
In order to circumvent the susceptibility of the powder part to ambient humidity, encapsulating at least parts of the powder component was considered. It was also considered to add desiccants to the powder part. Another approach was to package the final product or at least the powder part into a humidity tight foil blister. This kind of packaging, however, is quite expensive and produces waste after use, which is not desired. Encapsulating particles is often not easy and may affect the overall reactivity of the encapsulated powder. The same holds true for adding a desiccant. Thus, there is still room for improvement especially with regard to the requirements to be fulfilled with respect to modern dental materials. Some of the materials are not sufficiently storage stable or require the material to be stored under special conditions, e.g. by sealing the material in expensive packaging materials.
U.S. Pat. No. 4,376,835 (Schmitt et al.) describes a calcium aluminium fluorosilicate glass powder, wherein the calcium in the surface of the powder's particles is depleted. The glass powder may be prepared by surface treating calcium aluminium fluorosilicate powder particles with an acid which forms calcium salts, washing the calcium salts off the treated particles and drying the washed particles. Cements formed from the glass powder exhibit reduced periods of water sensitivity, while permitting sufficient time of processing.
U.S. Pat. No. 6,719,834 (Braun et al.) relates to a polyelectrolyte cement containing at least two reaction partners: a) at least one metal-cation-releasing compound and b) one or more polyelectrolyte capable of being converted into a solid state, wherein at least one of the polyelectrolytes is at least partially water soluble and wherein at least a part of the reaction partners (a) and/or (b) is coated with an organic surface-coating agent. The polyelectrolyte cement is stable in storage and can be easily mixed.
WO 2012/101432 relates to a mixture of a glass ionomer cement and zinc phosphate. Preferably, the composition comprises 40-95% by weight of fluorosilicate glass and 5-60% by weight of zinc oxide as acid-reactive components. The compositions are for use in the repair of human hard tissue, in particular as dental restorative materials and in orthopaedic surgery.
EP 2 011 469 describes a composition where hydroxyl apatite is added as a reactive component to glass ionomer cements.
EP 0 694 298 relates to the use of a preformed glass ionomer filler which comprises a powdery reaction product between a polyalkenoic acid and a fluoride glass. This filler can release fluoride ions. Whereas most examples refer to the use of the filler in resin containing, light-curing dental composition, there is also an example using this preformed glass ionomer in a carboxylate cement with Zinc oxide and Magnesium oxide as basic ingredients. Yet no examples were found with regards to the use of the pre-formed glass ionomer filler in conventional glass ionomers. Also it has to be understood, that the production of such a pre-formed glass ionomer fillers comprises several steps.
U.S. Pat. No. 5,318,929 discloses an apatite-containing glass ceramic, which can be used in particular in forming glass ionomer cement and biomaterials which improves manipulability and adhesion vis-a-vis known technical solutions of glass ionomer cements.
U.S. Pat. No. 4,738,722 describes a buffered glass ionomer cement for dental use, which contains as fillers fluoro boro phosphoro calcium alumino silicate, zinc oxide (5-20%) and titanium dioxide in place of about half the amount of zinc oxide.
U.S. Pat. No. 6,355,585 discloses a glass powder for glass ionomer cement having high mechanical strength, containing a glass powder for glass ionomer cement having a shape in which a major axis length is from 3 to 1,000 times a minor axis length, in a glass powder for glass ionomer cement. The composition of the glass powder described refers to an acid reactive fluoro alumino silicate glass.
U.S. Pat. No. 8,083,844 describes the use of hydroxyl apatite as filler in glass ionomer cements.
JP 2002-275017 (Noritake) describes a material for preparing dental glass ionomer cements. The powdery material comprises 10-50 wt.-% of fluoroaluminosilicate glass powder, less or equal than 10 wt.-% of a powder selected from certain oxides, with the balance of a powdery inert filler. Due to a reduced content of fluoroaluminosilicate glass powder (10 to 50 wt.-%), the glass ionomer cement is said to be excellent in temporarily adhesive and temporarily sealing use, i.e. has reduced mechanical properties. Compressive strength values in the range of less than 70 MPa are reported.
U.S. Pat. No. 5,520,922 (Gasser et al.) relates to a filling material for dental root canals comprising (A) 25-80 wt.-% glass ionomer cement containing (a) an aluminium fluorosilicate glass, (b) a certain polymeric polyacid, (c) water and (B) 25-75 wt.-% of a fluoride and/or oxide of heavy metal elements. In an example a cement powder is described containing 75 g of calcium tungstate, 25 g of calcium aluminium fluorosilicate glass and 4 g of pyrogenic silicic acid and pigments. The cement powder is mixed with an appropriate cement liquid resulting in a hardened product having a compressive strength of 90 MPa.